A material may have two or more display states differing in at least one optical property and for which a first display state may be changed to a second display state by applying an electric field to the material. A material of this type may be referred to herein as an “electro-optic” material. Pixels of a display device may incorporate an electro-optic material, providing a way for making the appearance of the pixels changeable. Although the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, luminescence, or pseudo-color. Pseudo-color refers to the reflectance of electromagnetic wavelengths outside the visible range. Pseudo-color may be used in displays intended for machine reading.
An optical state between two extreme optical states of a pixel may be referred to as a “gray state.” In addition, the two extreme optical states themselves may also be referred to as a gray state. The two extreme optical states need not be black and white. For example, the extreme states may be white and dark blue so that an intermediate gray state is a shade of blue. Nonetheless, extreme optical states may be referred to herein for convenience as black and white.
When an electric field of sufficient strength and proper direction is placed across an electro-optic pixel, the display state of the pixel changes. In a display device, the electric field may be created with a pair of electrodes. In particular, each pixel may be placed between a transparent, common electrode on the side of the pixel that is viewed by a user and an addressable electrode on the opposite side of the pixel. In one embodiment, the electric field required to change the state of electro-optic pixels of a display may be provided by an “active matrix” of non-linear elements, such as transistors or diodes. In an active matrix, each pixel is associated with at least one non-linear element. The pixels are arranged in rows and columns and each pixel is addressable according to its row and column position. In an exemplary active matrix display, the image may be updated one row at a time. A first voltage is applied to the common electrode. An activation voltage is applied to activate all of the non-linear elements for a particular row. A second voltage is applied to the column electrodes of desired pixels. The difference between the first and second voltages establishes the electric field that drives the particular pixels in the selected row to their new display states. Generally speaking, the magnitude and direction of the second voltage depends on the desired new display state. After an interval known as the “line address time” the column voltages are removed, the selected row is deselected, and the process may be repeated for the next row. The difference between the first and second voltages may be referred to as an “impulse,” as explained below. As further explained below, it is desirable with certain display devices to apply two or more impulses to a pixel when changing a pixel to a new display state.
When a pixel is driven to a new display state, it may maintain the new state after the electric field is removed, i.e., the display state may persist. Persistence refers to how long a pixel maintains a new display state after an electric field (or sequence of fields) is removed. Persistence may be defined with respect to line address time, the time associated with a sequence of electric fields, i.e. multiple line address times, an impulse, a sequence of impulses, a display refresh time, or in another suitable manner. If the display state of an electro-optic pixel persists, the pixel may be referred to herein as “bistable.” As one example, an electro-optic pixel for which a new display state persists for at least one order of magnitude longer than a typical liquid crystal display (LCD) pixel after being changed to the new display state may be considered bistable. As another example, an electro-optic pixel for which a new display state persists for at several times longer than a typical liquid crystal display (LCD) pixel after being changed to the new display state may be considered bistable. The term bistable may be used herein, for convenience, to refer to both pixels that have two display states and to pixels that have more than two display states, the later technically being multi-stable.
The term “impulse” may be used herein to mean the integral of voltage with respect to time. However, some bistable, electro-optic media act as charge transducers, and with such media an alternative definition of impulse, namely the integral of current over time (which is equal to the total charge applied) may be used. The appropriate definition of impulse should be used, depending on whether the medium acts as a voltage-time impulse transducer or a charge impulse transducer.
While the pixels of an impulse-driven electro-optic display may be driven directly from an initial display state to a final display state (“general grayscale image flow”), this technique may result in errors. For example, errors encountered in practice include:
(a) Prior State Dependence. With at least some electro-optic media, the impulse required to switch a pixel to a new display state depends not only on the current and desired display state, but also on the previous display states of the pixel.
(b) Dwell Time Dependence. With at least some electro-optic media, the impulse required to switch a pixel to a new display state depends on the time that the pixel has spent in its various display states. The precise nature of this dependence is not well understood, but in general, more impulse is required the longer the pixel has been in its current display state.
(c) Temperature Dependence. The impulse required to switch a pixel to a new display state depends heavily on temperature.
(d) Humidity Dependence. The impulse required to switch a pixel to a new display state depends, with at least some types of electro-optic media, on the ambient humidity.
(e) Mechanical Uniformity. The impulse required to switch a pixel to a new display state may be affected by mechanical variations in the display, for example variations in the thickness of an electro-optic medium or an associated lamination adhesive. Other types of mechanical non-uniformity may arise from inevitable variations between different manufacturing batches of medium, manufacturing tolerances and materials variations.
(f) Voltage Errors. The actual impulse applied to a pixel will inevitably differ slightly from that theoretically applied because of unavoidable slight errors in the voltages delivered by drivers.
General grayscale image flow also suffers from an “accumulation of errors” phenomenon. For example, assume that temperature dependence results in a 0.2 L* (where L* has the usual CIE definition:L*=116(R/R0)1/3-16,where R is the reflectance and R0 is a standard reflectance value) error in the positive direction on each transition. After fifty transitions, this error will accumulate to 10 L*. As a second example, assume that the average error on each transition, expressed in terms of the difference between the theoretical and the actual reflectance of the display is ±0.2 L*. After 100 successive transitions, the pixels will display an average deviation from their expected state of 2 L*. Deviations due to an accumulation of errors may be apparent to the observer.
Several types of electro-optic displays are known. One type of electro-optic display is a rotating bichromal member type as described, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791. Rotating bichromal member type displays use a large number of small bodies (typically spherical or cylindrical) which have two or more sections with differing optical characteristics, and an internal dipole. These bodies are suspended within liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate. The appearance of the display is changed by applying an electric field, rotating the bodies to various positions and varying which of the sections of the bodies is seen through a viewing surface. This type of display may be bistable.
Another type of electro-optic display uses an electrochromic medium. For example, a nanochromic film that includes an electrode formed at least in part from a semi-conducting metal oxide and two or more dye molecules capable of reversible color change attached to the electrode. Nanochromic films of this type are described, for example, in U.S. Pat. Nos. 6,301,038; 6,870.657; and 6,950,220. This type of display may be bistable.
Another type of electro-optic display is an electro-wetting display. See for application Ser. No. 10/711,802, filed Oct. 6, 2004 (Publication No. 2005/0151709). This type of display may be bistable.
Another type of electro-optic display is the particle-based electrophoretic display. Electrophoretic displays include two or more charged particles suspended in a fluid that may be made to move through the fluid under the influence of an electric field. The fluid is typically a liquid, but electrophoretic media may be produced using gaseous fluids. This type of display may be bistable. One commercial example is “electronic ink” available from E Ink Corp, Cambridge, Mass., a subsidiary of E Ink Holdings, Inc., Taiwan.
One type of electrophoretic display employs encapsulated electrophoretic media. Encapsulated electrophoretic media includes numerous small capsules. Each capsule includes an internal phase containing electrophoretically-mobile particles suspended in a liquid medium. A capsule wall surrounds the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer that may be positioned between two electrodes.
Another type of electro-optic display is the polymer-dispersed electrophoretic display. The polymer-dispersed electrophoretic media may be regarded as sub-species of encapsulated electrophoretic media. In a polymer-dispersed electrophoretic display, a continuous phase of a polymeric material is substituted for the walls surrounding discrete microcapsules of an encapsulated electrophoretic medium. The electrophoretic medium includes two or more discrete droplets of an electrophoretic fluid. The discrete droplets of electrophoretic fluid may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet. See, for example, U.S. Pat. No. 6,866,760.
A related type of electrophoretic display is a so-called “microcell electrophoretic display.” In a microcell electrophoretic display, the charged particles and the suspending fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, International Application Publication No. WO 02/01281, and published US Application No. 2002/0075556.
Although electrophoretic media are often opaque and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called “shutter mode” in which one display state is substantially opaque and one is light-transmissive. See, for example, U.S. Pat.s Nos. 6,130,774; 6,172,798; 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.
The bistable or multi-stable behavior of particle-based electrophoretic displays, and other electro-optic displays displaying similar behavior (such displays may be referred to as “impulse driven displays”), is in marked contrast to that of conventional LCDs. Twisted nematic liquid crystals are not bi- or multi-stable but act as voltage transducers, so that applying a given electric field to a pixel of such a display produces a specific gray level at the pixel, regardless of the gray level previously present at the pixel. Furthermore, LCDs are only driven in one direction (from non-transmissive or “dark” to transmissive or “light”). The reverse transition from a lighter state to a darker one is produced by reducing or eliminating the electric field. Generally, the gray level persists only while the electric field is applied. In addition, the gray level of a pixel of an LCD is not sensitive to the polarity of the electric field, only to its magnitude, and indeed for technical reasons LCDs usually reverse the polarity of the driving field at frequent intervals. In contrast, bistable electro-optic displays act, to a first approximation, as impulse transducers, so that the final state of a pixel depends not only upon the electric field applied and the time for which this field is applied, but also upon the state of the pixel prior to the application of the electric field.